Tunable cavity resonator with ramp shaped supports

ABSTRACT

A cavity for a hydrogen maser is disclosed consisting of three parts which provide highly stable mechanical and thermal expansion characteristics for the cavity and ease of tuning. The three parts which are made of a &#39;&#39;&#39;&#39;glass ceramic&#39;&#39;&#39;&#39; material having a very small thermal expansion coefficient ( Alpha of + OR 0.5 X 10 7 in/in/*C over 0*-38*C) include 1) a top plate, 2) a cylinder with three interrupted helical ramps at its bottom and 3) a base which includes a bottom plate and three ramp lugs on which the helical ramps of the cylinder rest when the cylinder is placed on the base with the bottom plate in the cylinder. Cavity tuning is achieved by rotating the cylinder and thereby raising or lowering it on the base, which results in changing the cylinder volume by changing the distance between the bottom and top plates.

United States Patent [191 Fletcher et al.

[ 1 Nov. 6, 1973 1 TUNABLE CAVITY RESONATOR WITH RAMP SHAPED SUPPORTS [76] Inventors: James C. Fletcher, Administrator of the National Aeronautics and Space Administration with respect to an invention of; Robert F.C. Vessot; Thomas E. Hoffman, both of Marblehead, Mass. 01945; Martin W. Levine, Manchester, Mass.

[22] Filed: Mar. 20, 1972 [211 Appl. No.: 235,962

[52] US. Cl. 333/83 R, 333/97 R 2,578,429 12/1951 Karplus 331/96 X 3,386,043 5/1968 Marcatili et al. 340/43 3,529,267 9/1970 Gunderson et al 333/83 T FOREIGN PATENTS OR APPLICATIONS 1,020,043 2/1966 Great Britain 333/83 R 967,331 10/1950 France ..333/83R Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Wm. H. Punter Attorney-Monte F. Mott et al.

[57] ABSTRACT A cavity for a hydrogen maser is disclosed consisting of three parts which provide highly stable mechanical and thermal expansion characteristics for the cavity and ease of tuning. The three parts which are made of a glass ceramic material having a very small thermal expansion coefficient (a of 105x10 in/in/C over 0-38C) include 1) a top plate, 2) a cylinder with three interrupted helical ramps at its bottom and 3) a base which includes a bottom plate and three ramp lugs on which the helical ramps of the cylinder rest when the cylinder is placed on the base with the bottom plate in the cylinder. Cavity tuning is achieved by rotating the cylinder and thereby raising or lowering it on the base, which results in changing the cylinder volume by changing the distance between the bottom and top plates.

8 Claims, 3 Drawing Figures ORIGIN OF INVENTION DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel features of the cavity of the present inven- The invention described herein was made in the per- 5 tion are shown in FIGS. 1 and 2, wherein like elements formance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to a hydrogen maser and, more particularly, to an improved cavity therefor.

2. Description of the Prior Art Atomic hydrogen masers have been developedand described in the literature. These masers have have proven to be useful both as spectroscopic tools and as frequency standards. As is appreciated by those familiar with the art the accuracy and stability of the output frequency of such a maser greatly depends on the stability of various critical elements, particularly the mechanical and thermal stability of the masers cavity. In' addition, the tuning of the latter is very criticalfor an accurate and stable output frequency. Although to date various cavity structures have-been designed none has adequately solved the stringent stability and tuning requirements.

OBJECTS AND SUMMARY OF THEINVENTION It is a primary object of the present invention toprovide an improved hydrogen maser cavity.

Another object of the present invention is to provide a hydrogen maser cavity which exhibits superior mechanical and thermal stability characteristics.

A further object of the present inventionis to provide an improved hydrogen maser cavity which is easily and precisely tunable, and which exhibits superior mechan-. ical and thermal stability characteristics.

These and other objects of the inventionare achieved by providing a three-part cavity structure made of a material which exhibits extremely low thermal expansion and superior mechanical stability characteristics. The three parts of the cavity include I) a top plate, 2) a cylinder with a plurality of interrupted helical ramps or threads at its bottom, and 3) a base which contains the bottom plate of the cavity and a plurality of ramp lugs, which engage the cylinders interrupted helical threads. The number of ramp lugs is thesame as the number of the interrupted helical ramps. Cavity tuning is achieved by rotating the cylinder and thereby raising or lowering it on the base which results in changing the volume of the cylinder between the top and bottom plates.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of the novel cavity of the present invention;

FIG. 2 is a side view of parts of the cavity; and FIG. 3 is a side cross-sectional view of the assembled cavity and the manner in which it is employed.

or parts are designated by like numerals. As stated briefly herebefore the cavity, designated by numeral 10, consists of three basic-parts. These include 1) a top plate 12 with several, e.g., three, integral stiffening webs 13, 2) a hollow cylinder 14 and 3) a base 16. The cylinder 14 has a multifold, e.g., three-fold interrupted helical ramp or thread at its bottom. The three ramps are designated in FIG. 1 by numerals 18. In FIG. 2 only two of the ramps are shown. The base 16 which is shown resting on a metal disc 19 includes the bottom plate20 of the cavity and a plurality, e.g., three ramp lugs 22, which engage the cylinders interrupted ramps 18. The base is strengthened by stiffening webs 24.

In practice the cylinder 14 rests on the base with the lugs 22 engaging the ramps 18, so that the bottom plate 20 is inside the cylinder, and the top plate 12 is placed at the top of the cylinder. Thus the volume of the cavity is defined by the cylinders inner diameter and the distance between the top and bottom plates. Cavity tuning is easily and precisely achieved by rotating the cylinder and thereby raising or lowering it on the base which in turn controls the distance of the top plate from the bottom plate, which is fixedly connected and is a part of the base.

The material from which thetop plate 12, the cylinder l4 and the base 16 are manufactured is chosen on the basis of its mechanical and thermal stability characteristics. At present materials are available which exhibit excellent mechanical and thermal stability characteristics. These materials are generally known in the art as glass ceramics. Some of these materials have a thermal expansion coefficient, known as a, with la] 50.5X 10" in in/in/C over a temperature range of -38C. Thus they are particularly adapted for fabrieating the cavitys parts. In one embodiment actually reduced to practice a material sold by Owens-Illinois and known as CER-VIT C-lOl was used with excellent results. This material is mentioned herein merely to recite one type of material which may be used, rather than to limit the invention thereto.

In practice the cavity is supported and secured within a metallic hold-down cylinder 25, shown in FIG. 3. Cylinder 25 is fastened at the bottom to the disc 19. A

Belleville spring assembly 26 is used to provide uniform downward pressure at the outer circumference of the top plate 12. The tension of the spring assembly is set by means of adjusting screws. 28 and a force distributing ring29 so that thermal movement of the cylinder 25 relative to that of the cavity material does not cause any appreciable change in the compressive force on the top plate 12 of the cavity. In FIG-3 the entire assembly is shown supported within a bell jar 30 is also secured to the bottom disc 19.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art and consequently it is intended that the claims be interpreted to cover such modifications and equivalents.

What is claimed is: 1

1. In a tunable cavity the arrangement comprising:

a hollow cylinder having first and second opposite ends;

a top plate closing said first end; and

a base member including a bottom plate insertable in said cylinder through said second end and support means for engaging the cylinder at said second end so that the distance between said top and bottom plates varies as said cylinder is rotated about its longitudinal axis while the cylinder is supported and engaged by said support means, said support means comprising n ramp lugs extending upwardly from said base member, n being an integer, and

said cylinder forms n interrupted ramps at said second end, with each interrupted ramp resting on one of said lugs whereby as said cylinder is rotated in a first direction the cylinder is lowered on the base and is raised therefrom when the cylinder is rotated in a second direction opposite said first direction, to thereby control the distance between said top and bottom plates, within said cylinder.

2. The arrangement as recited in claim 1 further including means for providing a uniform compressive force to the outer circumference of said top plate.

3. The arrangement as recited in claim 1 wherein each of said top plate, said cylinder and said base memher is of a material which has a thermal coefficient of expansion definable as a which is not more than "LO.5X1O" in/in/C over a preselected temperature range.

4. The arrangement as recited in claim 1 wherein n is not less than 3,

5. The arrangement as recited in claim 4 wherein each of said ramps is helically shaped.

6. The arrangement as recited in claim 4 wherein each of said top plate, said cylinder and said base member is of a material which has a thermal coefficient of expansion definable as 0: which is not more than :*-.0.5Xl0' in/in/C over a preselected temperature range.

7. The arrangement as recited in claim 5 further including means for providing a uniform compressive force to the outer circumference of said top plate.

8. The arrangement as recited -in claim 7 wherein each of said top plate, said cylinder and said base member is of a material which has a thermal coefficient of expansion definable as a which is not more than iO.5Xl0' in/in/"C over a preselected temperature range. 

1. In a tunable cavity the arrangement comprising: a hollow cylinder having first and second opposite ends; a top plate closing said first end; and a base member including a bottom plate insertable in said cylinder through said second end and support means for engaging the cylinder at said second end so that the distance between said top and bottom plates varies as said cylinder is rotated about its longitudinal axis while the cylinder is supported and engaged by said support means, said support means comprising n ramp lugs extending upwardly from said base member, n being an integer, and said cylinder forms n interrupted ramps at said second end, with each interrupted ramp resting on one of said lugs whereby as said cylinder is rotated in a first direction the cylinder is lowered on the base and is raised therefrom when the cylinder is rotated in a second direction opposite said first direction, to thereby control the distance between said top and bottom plates, within said cylinder.
 2. The arrangement as recited in claim 1 further including means for providing a uniform compressive force to the outer circumference of said top plate.
 3. The arrangement as recited in claim 1 wherein each of said top plate, said cylinder and said base member is of a material which has a thermal coefficient of expansion definable as Alpha which is not more than + or - 0.5 X 10 7 in/in/*C over a preselected temperature range.
 4. The arrangement as recited in claim 1 wherein n is not less than
 3. 5. The arrangement as recited in claim 4 wherein each of said ramps is helically shaped.
 6. The arrangement as recited in claim 4 wherein each of said top plate, said cylinder and said base member is of a material which has a thermal coefficient of expansion definable as Alpha which is not more than + or - 0.5 X 10 7 in/in/*C over a preselected temperature range.
 7. The arrangement as recited in claim 5 further including means for providing a uniform compressive force to the outer circumference of said top plate.
 8. The arrangement as recited in claim 7 wherein each of said top plate, said cylinder and said base member is of a material which has a thermal coefficient of expansion definable as Alpha which is not more than + or - 0.5 X 10 7 in/in/*C over a preselected temperature range. 